 <Window
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
  Height="200" Width="600" Title="Growing Label Font!">
  <StackPanel>
    <Label Content = "Interesting...">
      <Label.Triggers>
        <EventTrigger RoutedEvent = "Label.Loaded">
          <EventTrigger.Actions>
            <BeginStoryboard>
              <Storyboard TargetProperty = "FontSize">
                <DoubleAnimation From = "12" To = "100" Duration = "0:0:4" 
                  RepeatBehavior = "Forever"/>
              </Storyboard>
            </BeginStoryboard>
          </EventTrigger.Actions>
        </EventTrigger>
      </Label.Triggers>
    </Label>
  </StackPanel>
</Window>